Travelling wave tubes



7, 6 A. SAMUEL TRAVELLING WAVE TUBES Filed March 13, 1951 INVENTOR fiPn/z/R ASA/W051 6 7 AIITORNEY TRAVELLING WAVE TUBES Arthur L. Samuel, Poughkeepsie, N. Y., assignor to Sperry Rand Corporation, a corporation of Delaware Application March 13, 1951, Serial No. 215,291

3 Claims. (Cl. 315-35) This invention relates to improvements in electron discharge tubes of the travelling wave type, wherein a high frequency wave to be amplified is conducted along a slow wave electromagnetic propagating structure, such as a helix, in energy exchanging relationship with a stream of electrons which is directed along the structure at approximately the same velocity as the wave.

To obtain most efiective coupling between the travelling wave field and the electrons, it is necessary for the electron stream to pass very close to the conductive surfaces of the wave propagating structure, where the fields are most intense.

In the conventional helix type of travelling wave tube, some of the electrons in the stream do not go all the way through the helix, but strike the helix wire. This elfect may be aggravated when a high density electron beam is used, owing to space charge repulsion. Thus as tubes of this type are designed for higher power outputs, the heating of the helix by intercepted electrons becomes a major factor, and tubes have been made with the helix of tungsten wire, and operated at incandescence.

Other types of wave propagating devices, structurally better suited for external cooling than the helix, have been proposed and used. One example is the internally finned wave guide, or pill box arrangement. These generally have the characteristic of being narrow band devices, be cause the velocity of wave propagation varies considerably with variation in frequency, and hence the wave velocity matches the electron velocity only within a relatively narrow range of frequencies.

The principal object of the present invention is to provide an improved slow wave propagating structure for travelling wave tubes, wherein the above mentioned limitations on power capability and band width are avoided.

More specifically, it is an object of this invention to provide a slow wave propagating structure which may include a helix similar to those used heretofore, but which also includes means for shielding the helix from stray electrons which would otherwise strike it.

The invention will be described with reference tothe accompanying drawings, wherein Fig. 1 is a longitudinal section of a presently preferred embodiment, and Fig. 2 is a transverse section of the device of Fig. l in the plane II-II of Fig. 1.

The travelling wave tube shown in the drawing includes a tubular conductive sheath 4 provided at its ends with glass bulb members 10 and 16 which are sealed to the sheath and cooperate therewith to form a vacuum envelope. The bulb 10 contains electron beam-forming means including a cathode 9, a focussing electrode 11, and an accelerating electrode 12. The bulb 16 contains a collector electrode 17, All of the foregoing elements may be of known type and conventional design, and their function is to provide an approximately cylindrical stream of electrons flowing from the cathode to the collector.

The tubular sheath 4 contains and supports an internal tates Patent 2,758,242 Patented Aug. 7, 1956 conductive fin 6 which is in the form of a solenoid or helix extending from one end of the member 4 to the other. The fin 6 may be of oblong cross section as shown, with its longer dimension extending radially inward from the inner surface of the tubular sheath 4. The ends of the fin are secured to transverse discs 14 at the ends of the sheath 4. The discs 14 are provided with central openings for the passage of the electron stream.

A helix 3 of conductive wire is intercalated with the fin 6 as shown, with each turn of the helix 3 lying substantially midway between two successive turns of the fin. The helix 3 may be supported by a plurality of quartz rods 5 which extend lengthwise of the tube through holes in the fin 6 and the discs 14, in contact with several points on the outer periphery of each turn of the helix 3.

One end of the wire forming the helix 3 extends out tangentially through a hole in the sheath 4 to an external fitting which comprises coaxial inner and outer conductors 1 and 2, and is adapted to engage a mating fitting for coupling to a coaxial transmission line, not shown. The other end of the helix 3 is likewise coupled to a similar fitting designated generally by the reference number 18.

To dissipate heat produced in the operation of the tube, the sheath 4 may be provided with external radiating fins, or may be arranged for liquid cooling, as shown. In this case a coolant conducting pipe or tube 8 is disposed on the outer surface of the sheath 4 and water or other liquid may be circulated through it.

In the operation of the device, high frequency energy to be amplified is applied through a coaxial line to the coaxial input means 1, 2. Amplified energy is taken out through the output fitting 18. To obtain high power output capability, the beam forming means 9, 11, 12 may be designed and operated to produce a dense, high velocity stream. Preferably though not necessarily the fin 6 extends radially inward far enough to obscure the cathode from the wire helix 3.

Ideally, all of the electrons emitted by the cathode should go through to the collector. Actually, a certain proportion of the electrons diverge from their desired paths and are collected by the wave propagating device. In the described structure, most of the stray electrons strike the fin 6 rather than the helix 3. The heat generated by impact of the intercepted electrons is readily conducted away by the fin, and dissipated by the external cooling means. In prior art structures wherein stray electrons are intercepted principally by the helix, substantially all of the resulting heat must be dissipated by radiation from the helix, thus imposing a severe limitation on power handling capability.

The fin 6 does not function solely as a shield to prevent electrons from striking the helix 3, but also acts as a part of the wave propagating mechanism. It can be regarded as carrying a wave of its own, like the wave carried by the helix 3. The total wave travelling along the structure is the resultant of these two composite waves. The helical fin 6, together with the cylindrical outer sheath 4, may also be considered as an outer transmission line conductor formed as a helix and open on the inner surface of the helix to expose the inner conductor 3.

While it is possible that the helix 3 might be omitted, and the helically internally finned sheath used alone as the wave propagating device, this is not recommended in the practice of the present invention for several reasons. Certain theoretical considerations show that the helix and fin together will propagate waves at the same velocity throughout a wider band of frequencies than either the helix or the fin alone. Also, it is believed that the described structure provides stronger interaction between the wave and the electron beam than would be obtained with the helical fin alone. Furthermore, the intercalated helix and fin are adapted to be coupled to coaxial input and output lines without the introduction of large impedance discontinuities. This results from the fact that each adjacent pair of turns of the helical fin 6, together with the portion of the tubular conductor 4 lying between them, define a trough-shaped section partially enclosing the helix wire. The trough-shaped shield cooperates With the wire to act like a coaxial transmission line with one side left open, and the combination has approximately the same impedance characteristics as an ordinary coaxial line.

Since the helix 3 need not be made of heat resistant material such as tungsten, but may be copper or copper plated, it will exhibit low inherent loss. Center attenuation or sectionalization may be necessary to prevent oscillation under some conditions. This can be provided conveniently by plating one or two turns of the fin 6, near the center of the tube, with nickel or other lossy material. The ends of the fin near the discs 14 may be similarly terminated if necessary to prevent the formation of standing waves.

Since many changes could be made in the above construction and many apparently Widely ditierent embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A travelling wave tube including a slow wave electromagnetic propagating structure comprising a helical conductor, an electron gun adjacent one end of said helical conductor for producing and directing a stream of electrons along the longitudinal axis of said helical conductor and through said helical conductor in energy interchanging relationship therewith, said conductor surrounding said stream coaxially and being directly exposed to said stream, a helical shield member concentrically disposed with said helical conductor, said shield member having at least some convolutions with a radial extent greater than the thickness of said conductor, said convolutions extending between and inwardly beyond the turns of said wire to shield said conductor from said electron gun and thereby minimize interception of electrons by said conductor.

2. The invention set forth in claim 1, further including a tubular conductor coaxial with said helical conductor and surrounding and connected to said helical shield member, a coaxial transmission line terminal with its inner conductor connected to one end of said helix wire, another coaxial line terminal with its inner conductor connected to the other end of said helix wire, the outer conductors of said coaxial line terminals being connected to said tubular conductor.

3. The invention as set forth in claim 1, further including input and output transmission line means coupled to said shield member and the respective ends of said helical conductor, said helical conductor and shield member comprising a slow wave propagating structure in substantially matched impedance relationship with said transmission line coupling means.

References Cited in the file of this patent UNITED STATES PATENTS 2,300,052 Lindenblad Oct. 27, 1942 2,541,843 Tiley Feb. 13, 1951 2,578,434 Lindenblad Dec. 11, 1951 2,584,308 Tiley Feb. 5, 1952 2,651,686 Clavier et al Sept. 8, 1953 

